[libvirt] [PATCH for 5.3.0] storage_backend_rbd: Don't fail iterating over snapshots

[Jason Dillaman]

On Mon, Apr 29, 2019 at 8:57 AM Michal Privoznik <mprivozn at redhat.com> wrote:
>
> When cloning an RBD volume we try to find a snapshot which is
> not different to the image we're trying to clone. This boils down
> to calling rbd_diff_iterate() or rbd_diff_iterate2() on systems
> with newer ceph. These two are passed a callback -
> virStorageBackendRBDIterateCb() which mistakenly states that if
> a negative value is returned from the callback, the iteration is
> stopped but no error is reported. Well, that is not the case.
> I've looked into librbd sources and found that a negative value
> does mean stop of iteration but it also means an error. That's
> why any attempt to clone an RBD image results in the following
> error:
>
>   virsh # vol-clone --pool rbd_image_root coreos_2023 coreos00.disk
>   error: Failed to clone vol from coreos_2023
>   error: failed to iterate RBD snapshot coreos_2023 at base: Operation not
>   permitted
>
> Reported on libvirt-users list:
>
> https://www.redhat.com/archives/libvirt-users/2019-April/msg00060.html
>
> Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
> ---
>  src/storage/storage_backend_rbd.c | 7 +------
>  1 file changed, 1 insertion(+), 6 deletions(-)
>
> diff --git a/src/storage/storage_backend_rbd.c b/src/storage/storage_backend_rbd.c
> index f8c968e682..3fe188551c 100644
> --- a/src/storage/storage_backend_rbd.c
> +++ b/src/storage/storage_backend_rbd.c
> @@ -1031,14 +1031,9 @@ virStorageBackendRBDIterateCb(uint64_t offset ATTRIBUTE_UNUSED,
>  {
>      /*
>       * Just set that there is a diff for this snapshot, we do not care where
> -     *
> -     * When it returns a negative number the rbd_diff_iterate() function will stop
> -     *
> -     * That's why we return -1, meaning that there is a difference and we can stop
> -     * searching any further.
>       */
>      *(int*) arg = 1;
> -    return -1;
> +    return 0;
>  }

You could also tweak "virStorageBackendRBDSnapshotFindNoDiff" to
ignore the error if "diff > 0". That would still allow it to fail-fast
once a delta is found.

>  static int
> --
> 2.21.0
>
> --
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-- 
Jason